Exemplary embodiments pertain to the art of electronic engine controls (EEC) and, more particularly, to a method of monitoring an EEC to detect a loss of fuel screen open area.
Engines, particularly those used to power aircraft, rely on response characteristics of metered flow provided by a fuel control to provide proper dynamic performance. Monitoring response characteristics supports typical engine transients such as deceleration/acceleration and for less typical transients such as surge recovery and overspeed shutdown. In a typical fuel control algorithm, an electronic engine control (EEC) schedules fuel flow by establishing an opening of a metering valve (MV) having a flow window including a flow window area. The opening is based on signals from an electrical feedback device. The flow window area in conjunction with a pressure regulating valve (PRV) maintains a relatively constant pressure drop across the flow window to provide required fuel flow. The MV is typically controlled by a single stage servovalve (SSSV) that includes a torque motor (TM) that schedules the opening either with a jet pipe receiver/receiver ports or a flapper/nozzle(s) hydraulic circuit. The Velocity and direction of the MV is based upon a direction and level of current change from a null current.
Fuel controls generally include screens that are positioned to protect various hydraulic connections to the SSSV. During operation, contaminants in the fuel and/or fuel coking resulting from elevated fuel temperatures may block or clog the screens. Contaminants and/or coking blockage decrease an overall flow area of the screens. The decreased flow area reduces SSSV fuel flow used to position the MV. Difference screen locations within the SSSV hydraulic circuit determines whether a blockage will decrease MV velocity in an increasing, decreasing, or both increasing and decreasing flow directions.